1. Field of the Invention
The invention relates to a method of forming a cushion and especially to a method of secondary formation of a three-dimensional netted structure having undergone a primary formation.
More specifically, the invention is directed to a method for forming a cushion having excellent impact resistance and load bearing properties, which meets the specific needs of a large number of people in various fields. Various shapes and sizes of cushions are formed by the method. The method is applicable to seat cushions used for seating, lying, or riding, including in health appliances, and vehicle seats of, e.g., automobiles, motorbikes, bicycles, trains, ships and boats and aircraft, saddles for horse-riding, chairs, sofas and beds.
2. Description of the Related Art
Japanese Pat. No. 4350286 describes a conventional four-face forming method for forming a three-dimensional netted structure, wherein molten filaments made of a thermoplastic resin are extruded as the material or as the main material from a die with a nozzle end having a plurality of nozzle openings so as to naturally fall between partly submerged drawing machines. The drawing speed of the filaments is set to be lower than the falling speed. Two pairs of the drawing machines, wherein the drawing machines of each pair are placed to face each other, are arranged in a quadrilateral formation in the direction perpendicular to the extruding direction. The interval between the opposed drawing machines is set to be narrower than the width of the extruded filament assembly. All four faces of the outer periphery of the filament assembly are brought into contact with the drawing machines before and after the submerged portions of the drawing machines. The density of all the four faces of the outer periphery in a direction parallel to the extruding direction is made to be higher than the density of the other part excluding the four faces. This feature ensures that a post finishing step is not required and enhances the alignment.
Japanese Pat. Appl. No. 2003-2510898 proposes a method of forming a cushion wherein a three-dimensional structure is placed in a female mold. The three-dimensional structure is made from solid and/or hollow continuous filaments and/or short filaments with random loops or curls made of at least a thermoplastic resin and brought into contact with one another to be tangled with voids at a specific bulk density. The female mold and/or the three-dimensional structure are heated to a specific temperature required for thermally softening the three-dimensional structure. The female mold is mated with a male mold with the three-dimensional structure inbetween and clamped down. The three-dimensional structure is allowed to cool down and cure. The cushion is formed to have a volume corresponding to the stroke of the male mold mating with the deep-drawing female mold.
The method described in Japanese Pat. Appl. No. 2003-2510898 is easier than secondary processing of urethane foam and meets the more specific needs than the secondary processing of urethane foam. The compression molding method enables formation of any original shape conforming to the individual build, thus providing high-value-added products. The thickness of the cushion is controllable with only one mold. This can satisfy the diverse requirements for the characteristics of various products without providing a wide variety of molds. For example, the longer stroke produces the thinner cushion, while the shorter stroke produces the thicker cushion. This facilitates production of the cushion and meets diverse requirements for the characteristics of various products, such as custom-made cushions. Additionally, the spring-like characteristics may be controlled freely by changing the diameter, the material, the denier, the bulk density or the porosity of the filaments of the three-dimensional structure. While the spring characteristics of the material are unchanged, squeezing by compression molding changes the cushioning effect and varies the load distribution.
The method described in Japanese Pat. No. 4350286, however, requires post-processing by secondary formation according to the requirements of the materials to be produced, for example, to make the rounded edge or to form a recess for chairs. Such secondary formation is required especially for the flame-retardant seat cushion for aircraft to form a recess and make the rounded or flat edge corners. Such secondary formation is extremely complicated and troublesome. On the other hand, in the method described Japanese Pat. Appl. No. 2003-2510898 the three-dimensional netted structure is heated and thermally softened, the mold is closed for compression molding, and then cooled down, e.g., with water, to cure the three-dimensional netted structure, and, thereafter, the three-dimensional netted structure is removed from the mold. The mold is large and thereby needs a significantly long time to repeat the heating and the cooling steps. The three-dimensional netted structure cannot be removed from the mold until the temperature of the mold is sufficiently low. This increases production time and lowers production efficiency.